Method for forming composite parts from volatile-emitting materials using breathable tooling

ABSTRACT

A method and apparatus for curing composite materials that emit large volumes of volatiles during processing. The apparatus includes tooling having a rigid forming surface contoured to the shape of the formed composite part. A plurality of passages extend from the forming surface through the tooling. The passages are in fluid contact with an exhaust port. A composite workpiece is laid up and placed adjacent the forming surface. The apparatus includes a heater that heats the composite workpiece to temperatures at which volatiles are emitted. An evacuation system draws the volatiles away from the composite workpiece through the passages in the tooling and out of the exhaust port. One embodiment of tooling according to the invention is used to form a sine wave spar. The tooling includes upper and lower tool inserts upon which U-shaped composite channels are formed. The upper and lower tools are placed adjacent to each other to form the web of the spar. Cap strips are placed on either side of the joined U-shaped channels. Side rail tools are then placed adjacent the cap strips. A breather cloth is placed over the tools and the entire assembly is placed within a vacuum bag. The assembly is then placed within an autoclave and connected to the vacuum exhaust of the autoclave. During processing, volatiles are drawn away from the composite workpiece through the passages in the tooling using the vacuum exhaust.

REFERENCE TO RELATED APPLICATIONS

This application is a divisional application based upon U.S. patentapplication Ser. No. 08/468,807, filed Jun. 6, 1995, now U.S. Pat. No.5,709,893.

FIELD OF THE INVENTION

The invention relates to a method and apparatus for forming and curingcomposite materials, and more particularly to the forming and curing ofcomposite materials that produce large amounts of volatiles duringprocessing.

BACKGROUND OF THE INVENTION

The use of high strength fiber reinforced composite materials in themanufacture of aircraft and other lightweight structures has increasedsteadily since the introduction of such materials. Composite materialshave a high strength-to-weight ratio and stiff-ness. These propertiesmake composite materials attractive for use in the design of lightweightstructures. Some of the drawbacks to using composite materials have beentheir relatively high fabrication costs and low damage tolerance.Generally, it has been difficult to produce parts formed of highstrength composite materials at the same cost and having the same damagetolerance as comparable metal parts. Another disadvantage of compositematerials in some applications is their relatively low temperaturetolerance. Most widely used high-strength fiber reinforced compositematerials are not usable above 300° F.

Recent research has focused on the development of composite materialswith increased damage tolerance that can withstand higher temperatures.A number of promising composite materials use toughened epoxy orthermoplastic matrix systems. In addition to having increased damagetolerance, a number of these promising materials can withstand highertemperature environments than past thermoset materials. For example, anumber of promising materials, including DuPont's K-IIIB™, G.E.'sULTEM™, and ICI's APC-HTA™ have glass transition temperatures in therange of 400° F. to 500° F., as compared to past epoxy-based compositesglass transition temperatures in the 300° F. range.

Although new composite systems have increased damage tolerance and canwithstand higher temperatures, they require more stringent processingparameters. Some of these new composite materials must be processed attemperatures in the 600-800° F. range and at pressures of 100-300 psi.In addition, several of the new composite systems emit large quantitiesof gaseous volatiles during processing. For example, DuPont's AvimidK-IIIB thermoplastic (“K-IIIB”) material emits gaseous volatilesincluding water vapor, ethanol gas, and N-Methyl peryladone (“NMP”)during processing.

FIG. 1 is a graphical representation of the gaseous volatiles emittedfrom K-IIIB during processing. In FIG. 1, both weight loss andderivative weight are plotted along the x-axis, and temperature isplotted along the y-axis. As illustrated, a large quantity of water,ethanol, and NMP are emitted as the temperature increases during theprocessing cycle. The majority of the NMP is released before the timethe temperature reaches approximately 600° F. The majority of the waterand ethanol within the K-IIIB is released before the material reachesapproximately 800° F.

The high temperatures and pressures required for processing and theoff-gassing of volatiles increases the difficulty in processing the newmaterials. In order to produce quality void-free parts, it is necessaryto draw off the volatiles during curing. Failure to draw off thevolatiles during processing results in voids or areas of porosity in theformed composite parts.

Past methods used to draw off volatiles limit the ability to fabricatecomplex parts from volatile-producing materials. Currently, parts formedof volatile-producing materials are fabricated by laying up the materialon the forming surface of a shaped mandrel. The forming surface of themandrel has the contour of the completed part. A porous cloth breathingmaterial is placed on the side of the composite material opposite theforming surface. The tool, composite material and cloth breathingmaterial are enclosed within a sealed vacuum bag and placed in anautoclave. As the temperature of the tool and composite material areelevated, gaseous volatiles are emitted. These volatiles are drawn awayfrom the composite material through the cloth breathing material and outthrough the vacuum bag.

The use of cloth breathing materials allows quality parts to befabricated when access is available to at least one side of thecomposite material during processing. Access to one or more sides of thecomposite material is generally available only when it is not necessaryto maintain dimensional tolerances on more than one side of the partfabricated. However, if dimensional tolerances must be maintainedthroughout the part, it is generally not possible to use a clothbreathing material adjacent the composite material during processing.

In complex parts or parts requiring tight dimensional tolerances,matched tooling is generally used. When matched tooling is used, toolshaving rigid forming surfaces are placed in contact with most or all ofthe surfaces of the composite material during processing. Thedimensional tolerances of the formed part are determined by the shape ofthe forming surfaces on the tools.

For example, matched rigid tooling is generally used in the fabricationof I-beam and sine wave spars. The dimensional tolerances on both thetop and bottom caps of such spars must be maintained, thus requiring theuse of rigid matched tooling. Tooling having rigid forming surfaces arealso used to form the webs of the spars to ensure proper consolidationof the composite material. When rigid matched tooling is used, volatilesemitted become trapped within the composite material during processing.These trapped volatiles create voids and areas of porosity in thecompleted part.

In an attempt to withdraw volatiles when using matched tooling, theinventors placed cloth breathing materials between the compositematerial and the rigid forming surface of the tooling. The use of clothbreathing materials between the tool and the composite material createda number of problems. The cloth breathing material made it difficult,and in most circumstances impossible, to control dimensional toleranceson the surfaces of the formed composite part. The use of cloth breathingmaterials also inhibited the matched tooling from closing or createdpressure variations within the tooling during processing. The clothbreathing materials also became pinched off or were filled with resinduring processing, producing pressure variations. Such pressurevariations resulted in resin-rich, resin-poor, or porous regions withinthe formed composite part.

As can be seen from the above discussion, there exists a need for animproved method and apparatus for forming and curing volatile producingcomposite materials. The present invention is directed toward fulfillingthis need.

SUMMARY OF THE INVENTION

The present invention is a method and apparatus for forming compositeparts from composite materials that produce large volumes of volatilesduring processing. The invention allows volatiles to be drawn away fromthe composite material during processing, while allowing dimensionaltolerances to be maintained.

In one embodiment of a method according to the invention, volatileproducing composite materials are laid up to form a composite workpiece.The composite workpiece is placed adjacent a tool having a rigid formingsurface. A plurality of passages extend from the forming surface throughthe tool. The passages are in fluid contact with an exhaust port. Afterbeing placed adjacent the forming surface, the composite workpiece isheated to a temperature at which volatiles are emitted. These volatilesare removed from the composite workpiece through the passages and theexhaust port.

In accordance with other features of the invention, the passages in thetool may be formed by mechanically drilling, laser drilling or otherwiseforming passages in the structure of the tool. Alternatively, thepassages may be formed by forming the tool of a porous material. In oneembodiment of the invention, the tool is formed of porous graphite or asintered metal.

In one apparatus according to the invention, a tool having a rigidforming surface is provided. The tool includes a plurality of passagesextending from the forming surface through the tool. The passages are influid contact with an exhaust port. A composite workpiece is placedadjacent the forming surface. The apparatus also includes a heat sourcethat heats the composite workpiece to a temperature at which volatilesare emitted. The volatiles are removed from the composite workpiecethrough the use of an evacuation system that draws the volatiles throughthe passages and out of the exhaust port.

In accordance with other aspects of the invention, the passages have adiameter of between approximately 0.040″ and 0.062″. The tools can beformed of a gas impermeable rigid material having a plurality ofpassages formed by drilling, chemical etching, etc. Alternately, thetools can be formed of a rigid, porous material that allows gaseousvolatiles to pass through the passages within the porous material.

In accordance with still further aspects of the invention, the tools arehollow. The passages extend from the forming surface of the tool into acavity within the hollow tool. The cavity is filled with a porousbreather material.

In accordance with still other aspects of the invention, the compositematerial and tools are covered by a cloth breather material and areenclosed within a vacuum bag. The volatiles are withdrawn through thepassages in the tool, through the breather material and out through theexhaust port.

The invention allows high-quality composite parts to be formed ofcomposite materials that emit a large volume of volatiles duringprocessing. The invention is not limited to composite parts havingsimple geometries. Nor is the invention limited to the formation ofcomposite materials in which dimensional tolerances on only one surfaceof the composite part are important. The invention may be used withautoclave, press or other methods of processing.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing aspects and many of the attendant advantages of theinvention will become more readily appreciated as the same becomesbetter understood by reference to the following detailed description,when taken in conjunction with the accompanying drawings, wherein:

FIG. 1 is a graph of volatiles produced as a function of temperature,wherein weight loss and derivative weight are plotted along the y-axisand temperature is plotted along the x-axis;

FIG. 2 is a perspective view of a sine wave spar formed in accordancewith a method of the invention;

FIG. 3 is a cross-sectional view of sine wave spar tooling formed inaccordance with the invention;

FIG. 4 is a perspective view of the upper and lower tool inserts andcomposite subassemblies used to form the sine wave spar;

FIG. 5 is a partially exploded view of the lower tool insert and baseplate;

FIG. 6 is a partially exploded view of the upper and lower tool insertsand base plate;

FIG. 7 is a partially exploded view of the upper and lower tool insertsand radius fillers;

FIG. 8 is a partially exploded view of the upper and lower tool insertsand cap strips;

FIG. 9 is a partially exploded view of the upper and lower tool insertsand side rail tools;

FIG. 10 is a perspective view of the assembled sine wave spar tooling;

FIG. 11 is a perspective view of the sine wave spar tooling afterbagging; and

FIG. 12 is a cross-sectional view of another embodiment of sine wavespar tooling according to the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The present invention is a method and apparatus to form and curecomposite materials that produce large quantities of volatiles duringprocessing. The invention is described below with respect to a preferredembodiment used to form a sine wave spar. However, the method andapparatus of the invention may be used to form a wide variety ofcomposite structures. The preferred embodiment is also described withrespect to the use of DuPont's K-IIIB material system. However, theinvention may be used with other composite material systems.

FIG. 2 illustrates a sine wave spar 18 fabricated using a breathabletooling concept (FIG. 3) according to the invention. As best illustratedin FIG. 2, the sine wave spar 18 is an I-beam having a sine wave centralweb 20 that extends between left and right spar caps 22 and 24,respectively.

In the preferred embodiment, it is important to maintain dimensionaltolerances on the surfaces of the spar caps 22 and 24 and central web20. In order to maintain such tolerances, tooling having rigid formingsurfaces are used. As described below, the rigid forming surfaces areplaced adjacent the composite material and define the dimensions of thesine wave spar 18.

The sine wave spar 18 is fabricated using preformed subassemblies thatare joined together in an assembly process to form a compositeworkpiece. In the preferred embodiment, the subassemblies used to formthe composite workpiece include lower and upper U-shaped channels 36 and38 (FIG. 4), left and right radius fillers 40 and 42, and left and rightcap strips 44 and 46.

The lower and upper U-shaped channels 36 and 38 are formed by laying upcomposite material over lower and upper 30 and 32 rigid tool inserts(FIG. 4). Each tool insert 30 and 32 is generally rectangular andincludes a rigid forming surface having a sine wave contour 34. Thecomposite material is laid up on the forming surfaces so that it followsthe sine wave contour 34 and bends over the opposing edges of the toolinserts to form the lower and upper U-shaped channels 36 and 38. Thecomposite material may be placed on the tool inserts 30 and 32 usinghand lay-up procedures, automated tape laying, or other fabricationmethods.

The left and right cap strips 44 and 46 (FIG. 4) are formed by laying upstrips of composite material on a flat surface. The strips may be laidup by hand, using an automated tape laying machine, or using otherfabrication methods. The left and right radius fillers 40 and 42 areformed by hand lay-up, pultrusion, or other fabrication methods. Methodsfor laying up composite materials over tools and for fabricating radiusfillers are well known by those of ordinary skill in the art.

After the lower and upper U-shaped channels 36 and 38 are fabricated,the lower tool insert 30 is placed on a flat gage sheet 50 (FIG. 5). Thegage sheet 50 is mounted on a rigid rectangular base plate 52. The baseplate 52 has a larger surface area than the gage sheet 50 and supportsthe sine wave spar tooling during processing, as described below.

The lower tool insert 30 is indexed in a predetermined location on thegage sheet 50 and base plate 52 using a plurality of indexing pins 54(FIG. 5) in a manner well known in the art. The indexing pins 54 extendupward from the surface of the base plate 52 and engage recesses (notshown) in the lower surface of the lower tool insert 30. After the lowertool insert 30 is indexed, the upper tool insert 32 is inverted andplaced on top of the lower tool insert 30 and aligned (FIG. 6) in amanner well known in the art. For example, indexing pins (not shown)extending upward from the lower tool insert 30 can engage recesses (notshown) in the upper tool insert 32.

As the lower and upper tool inserts 30 and 32 are aligned, the lower andupper U-shaped channels 36 and 38 are placed in contact with each other,forming the central web 20 (FIG. 2) of the sine wave spar 18. Due torounded edges on the lower and upper tool inserts 30 and 32, atriangular gap is formed between the intersection of the lower and upperU-shaped channels 36 and 38. These triangular gaps are filled using leftand right radius fillers 40 and 42, respectively (FIG. 7). The radiusfillers 40 and 42 are placed in the triangular gaps and secured using aheat gun or another method as known by those of ordinary skill in theart. After the radius fillers 40 and 42 are in place, tfhe left andright cap strips 44 and 46 are placed in contact with the sides of theU-shaped channels 36 and 38 and radius fillers 40 and 42 (FIG. 8).

Left and right side rail tools 60′ and 62 (FIG. 9) having rigid formingsurfaces are then placed adjacent the left and right cap strips 44 and46. The left and right side rail tools rest upon the base plate 52. Therigid forming surfaces of the side rail tools 60 and 62 define thedimensions of the exterior surfaces of the left and right spar caps 22and 24 (FIG. 2).

Depending upon the application, the left and right side rail tools 60and 62 may be allowed to float, i.e., move in and out toward and awayfrom the upper and lower tool inserts 30 and 32 during processing asdescribed below. Alternately, the left and right side rail tools 60 and62 may be indexed into a predetermined position by indexing pins 55(FIG. 9) extending upward from the base plate 52. The indexing pins 55are received within recesses (not shown) in the lower surface of theside rail tools 60 and 62.

After the side rail tools 60 and 62 are in place, end tools 64 and 66are placed on the opposing ends of the lower and upper tool inserts 30and 32 (FIG. 9). The end tools 64 and 66 have rigid forming surfacesthat help to define the opposing ends of the sine wave spar. The endtools 64 and 66 are indexed in place by indexing pins 57 that extendupward from the base plate 52. The indexing pins 57 engage recesses (notshown) in the lower surface of the end tools 64 and 66.

In alternate embodiments of the invention, other tooling concepts couldbe used to fabricate the sine wave spar 18. For example, theconfiguration of the tools or the arrangement of the indexing pins couldbe changed. Different tooling concepts and methods of indexing tools arereadily understood by those of ordinary skill in the art.

As illustrated in FIG. 10, the lower and upper tool inserts 30 and 32,side rail tools 60 and 62, and end tools 64 and 66 surround thecomposite material forming the sine wave spar 18. The rigid formingsurfaces of the tools define the dimension of the formed spar. Tomonitor the processing parameters, one or more thermocouples 70 can beincorporated in the composite material or tools. The thermocouples 70can be connected to appropriate monitoring equipment to monitor thetemperature of the tools or composite material during processing.

As illustrated in FIG. 3, after the tooling is in place, it is coveredby a cloth breather material 72. The cloth breather material 72partially covers the surface of the base plate 52 and extends over thetop of the left and right side rail tools 60 and 62, end tools 64 and66, and upper tool insert 30. A vacuum bag 74 (FIGS. 3 and 11) is thenplaced over the top of the cloth breather material 72 and sealed to thebase plate 52 along its edges 76 in a manner well known in the art.

As discussed in the background section, it is important when processingvolatile producing composite materials to allow a pathway for thevolatiles to be withdrawn. In accordance with the present invention, oneor more of the tools are formed with passages that allow gaseousvolatiles to pass through the tools during processing. The gaseousvolatiles are then withdrawn out of the vacuum bag as described below.

In the preferred embodiment, both the upper tool insert 32 and the leftand right side rail tools 60 and 62 include a plurality of holes orpassages 80 (FIG. 3). The passages 80 extend from the forming surfacesof the upper tool insert 32 and side rail tools 60 and 62 through thethickness of the tools.

The upper tool insert 32 is U-shaped and includes a U-shaped cavity inits upper surface. The cavity is filled with a porous breather material82 (FIG. 3). The left and right side rail tools 60 and 62 are hollow andare also filled with porous breather material 82. The porous breathermaterial 82 can be any material that allows gaseous volatiles to passthrough the material while withstanding the high temperatures andpressures placed on the tooling during processing. In the preferredembodiment, the porous breather material 82 is formed of fiberglassbreather cloth.

In the preferred embodiment, the upper tool insert is formed with aU-shaped cavity and the side rail tools 60 and 62 are hollowed out inorder to decrease the weight and costs of the tools. Generally, porousmaterials suitable for use as breather materials 82 are less dense andthus lighter weight than the materials from which the tools arefabricated. Such breather materials 82 are also generally less expensivethan the materials used to form the tools. In some applications, thetools can be large and quite heavy. Thus, there are advantages to usingrigid tools filled with lighter weight breather materials as describedabove.

In alternate embodiments of the invention, solid tools could be used. Insuch embodiments, the passages 80 can pass through the entire thicknessof the tool. In such applications, the passages through the tools wouldprovide a path through which the gaseous volatiles could pass throughthe tools into the cloth breather material 72.

In the preferred embodiment, the tools 30, 32, 60 and 62 are formed ofInvar 42. Invar 42 is preferred because it has a coefficient of thermalexpansion compatible with many high temperature composite materials.However, in alternate embodiments, the tools may be formed of othersuitable materials including other metals, ceramics or composites.

In the preferred embodiment, the passages 80 are formed by mechanicallydrilling holes through the thickness of the tools. However, laserdrilling, chemical etching, casting, or other methods could be used toform the passages.

The passages 80 extend through the thickness of the tools 32, 60 and 62,approximately normal to the rigid forming surfaces of the tools. Thepassages 80 allow gaseous volatiles emitted during processing to flowinto the porous breather material. The porous breather material 82 inturn allows the gaseous volatiles to flow into the cloth breathermaterial 72 covering the tools.

The cloth breather material 72 may be placed in direct contact with thebreather material 82 (FIG. 3), as in the case of the breather materialin the upper tool insert 32. Alternately, as in the case of the siderail tools 60 and 62, passages may be drilled through the sides of thetools to allow gases to pass out of the breather material 82, throughthe tool, and into the cloth breather material 72.

The gaseous volatiles are withdrawn out of the cloth breather material72 and vacuum bag 74 through exhaust ports 100. The exhaust ports 100are located on the left and right sides of the left and right side railtools 60 and 62 (FIG. 3). The exhaust ports 100 extend through thethickness of the base plate 52 and open into the vacuum bag 74underneath the cloth breather material 72. The periphery of each exhaustport 100 is sealed by a seal 102 in a manner well-known in the art.

In the preferred embodiment, the vacuum bagged tooling assembly 16(FIGS. 3 and 11) is placed within an autoclave (not shown) and theexhaust ports 100 are attached to the vacuum exhaust (not shown) of theautoclave. The autoclave is then closed and the interior of theautoclave is pressurized and heated in accordance with the processingrequirements of the composite material used.

During processing, a vacuum is placed on the exhaust ports 100 toevacuate the interior of the vacuum bag 74. The vacuum may be placed onthe exhaust ports through the use of any appropriate vacuum device suchas the vacuum pump within the autoclave. The combined pressure producedby the evacuation of the vacuum bag 74 and the pressurization of theautoclave presses the upper tool insert 32 downward and the side railtools 60 and 62 inward. As the tools are pressed downward and inward,the rigid forming surfaces of the tools consolidate the compositematerial.

As the temperature of the composite material is elevated, gaseousvolatiles are emitted. The gaseous volatiles flow through the passages80 in the tools 32, 60 and 62 into the breather material 82. The gaseousvolatiles then flow through the breather material 82 into the clothbreather material 72. The gaseous volatiles are then withdrawn from thevacuum bag 74 and cloth breather 72 through the exhaust ports 100.

Although the preferred embodiment of the invention is described withrespect to the use of an autoclave, other apparatus could be used toprocess the composite material. For example, the tooling assembly 16could be heated through the use of heating elements embedded within thetools, inductive heating, or through the use of other heating methodsand apparatus. Similarly, although in the preferred embodiment, anautoclave is used to apply a consolidation pressure to the compositeworkpiece, a mechanical apparatus such as a press could also be used.

The number of, diameter and spacing of the passages 80 is a function ofseveral variables, including the composite material used, the quality ofthe surface finish desired, ease of cleaning desired, and cost.Increasing the number of passages used increases the associatedfabrication costs. Using passages with greater diameters makes it easierto clean the passages of any obstructions caused by resin flowing intoand blocking the passages. However, increasing the diameter of thepassages also allows more resin to flow into the passages, thus possiblyresulting in a markoff that detracts from the surface finish of theformed part. The size and number of passages used is also directlyrelated to the quantity of volatiles produced by the composite materialduring processing. Composite materials that produce larger volumes ofvolatiles require a higher percentage of the surface area to be taken upby passages to remove the volatiles.

In the preferred embodiment, DuPont's K-IIIB material system was used tofabricate the sine wave spar 18. Based on results using the K-IIIBmaterial system, it was found most advantageous for the percentage ofopen area containing passages to the overall tool forming surface areato be approximately 1.44%. However, the percent open area is influencedby a number of factors including the material system used. Therefore, inother embodiments, the percentage of open area used could change.

It was also found that passages 80 having diameters betweenapproximately 0.046″ and 0.062″ are most advantageous. Passages 80having diameters larger than approximately 0.062″ produced unacceptablemarkoff on the cured part's surface finish. Passages 80 with diametersbelow approximately 0.046″ were found to be too difficult to clean afterprocessing. The small diameter of such passages prevented cured resinthat flowed into the passages during processing from being easilyremoved.

As composite materials pre processed, there is a tendency for some resinto enter the interior of the passages 80. In some cases, the resin cureswithin the passages and blocks or obstructs the flow of gaseousvolatiles through the passages. Therefore, it is important that thepassages 80 be inspected and cleaned after a part is fabricated. Thepassages 80 can be cleaned using solvents, mechanical cleaning means,high pressure air systems, etc. Problems created by resin flowing intothe passages can be minimized by properly tailoring the diameter anddistribution of the passages. The passages should allow uninhibitedvolatile evolution while keeping resin bleed to a minimum.

Although the preferred embodiment of the invention uses tools formed ofInvar 42, tools formed of other materials may also be used. In someapplications, tools formed of ceramics, monolithic graphite, compositematerials, sintered bronze or copper, or other materials may be used. Inaddition, although the passages 80 in the preferred embodiment areformed by drilling, boring, etc., the passages could also be formed byother means. For example, the passages could be formed integrally withinthe tools by casting, or by forming the tools of porous material such asa sintered metal, monolithic graphite or a porous ceramic.

FIG. 12 illustrates the use of tooling formed of monolithic graphite. InFIG. 12, the monolithic graphite tooling is configured in the sametooling concept as that described with respect to the Invar 42 toolingof the preferred embodiment described above. The tooling concept andvacuum bagging and curing are similar to that described above withrespect to the preferred embodiment and may be understood by referenceto the discussion of the preferred embodiment.

The monolithic graphite tooling includes lower and upper tool inserts130 and 132, and left and right side rail tools 160 and 162. The upperand lower tool inserts and left and right side rail tools are configuredin a manner similar to that described above with respect to thepreferred embodiment. Composite materials are laid up over the lower andupper 130 and 132 tool inserts. The tool inserts are then assembled toform the web 22 of the sine wave spar. In a manner similar to thatdescribed above with respect to the preferred embodiment, radius fillersand cap strips are laid up and assembled to form the left and right sinewave spar caps 22 and 24.

The left and right side rail tools 160 and 162 are then placed on thebase plate 152 adjacent the spar caps 22 and 24. A gage plate 163 isthen placed over the top of the upper tool insert 132. A cloth breathermaterial 72 and vacuum bag 74 are then placed over the top of the tools.The vacuum bag 74 is sealed around its periphery edges 76 in a mannersimilar to that described above with respect to the preferredembodiment. The vacuum bagged graphite tooling assembly is then placedwithin an autoclave and processed in accordance with the processingparameters of the composite material used.

During processing, the gaseous volatiles produced by the compositematerial pass through the thickness of the monolithic graphite toolsinto the breather cloth 72. The gaseous volatiles are then withdrawn outof the breather cloth 72 and vacuum bag 74 through the exhaust ports 100in a manner similar to that described above with respect to thepreferred embodiment.

Monolithic graphite tooling is advantageous in some applications due toits lower cost versus comparable drilled Invar 42 tooling. However,monolithic graphite tooling is more brittle and is thus more prone todamage in a limited number of processing cycles. Due to its brittlenature, monolithic graphite tooling is advantageously used inapplications where a limited number of parts are produced.

Unlike the drilled Invar 42 tooling described above, monolithic graphitetooling is porous by nature. The porosity of the monolithic graphitetooling allows gaseous volatiles produced by the composite material tomigrate through the natural passages in the tooling. In addition to itsbrittle nature, the use of monolithic graphite tooling is limited by theporosity within the tooling. During each processing cycle, some resinenters the monolithic graphite tooling blocking the passages within thetooling. Over time, a sufficient quantity of the passages become blockedto prevent the flow of gaseous volatiles through the tooling. Thus,monolithic graphite tooling may be used for a limited number ofprocessing cycles.

The present invention allows complex parts to be formed from compositematerials that produce large quantities of, volatiles. The rigid formingsurfaces of the tooling of the invention allow part tolerances betightly maintained while the passages through the tooling allow gaseousvolatiles emitted during processing to be removed.

While the preferred embodiment of the invention has been illustrated anddescribed, it will be appreciated that various changes can be madetherein without departing from the spirit and scope of the invention.

1. A method for forming a part from a composite material containing aresin that emits volatiles during processing, comprising the steps of:laying up the composite material on a rigid forming surface of a moldtool to form a composite workpiece, the tool maintaining dimensionaltolerances and having a plurality of volatile-egress passages that arein fluid contact with an exhaust port, overlie substantially the entiresurface of the workpiece, and-extend from the forming surface of thetool through the tool to allow volatiles emitted from the workpiece tovent from the tooling through the exhaust port; heating the workpiece toa temperature at which the composite material emits volatiles to curethe resin on the surface to form the part; removing the volatiles, whilethe resin is curing, through the passages and the exhaust port; removingthe part from the tool; and after removing the part, cleaning thepassages in the tool to remove resin that enters and blocks or obstructsthe passages during removing the volatiles.
 2. The method of claim 1,further comprising the step of applying a consolidation force on theworkpiece through a matching tool operatively associated with the moldtool while heating the workpiece.
 3. The method of claim 1, furthercomprising placing a cloth breather material and vacuum bag over theworkpiece, prior to heating the workpiece, the cloth breather materialbeing positioned to direct the emitted volatiles to the exhaust port. 4.The method of claim 1 further comprising forming the tool from a rigidporous material.
 5. The method of claim 1 wherein the composite materialis a prepreg having reinforcing fibers impregnated with a thermoplasticresin, and wherein The volatiles include a solvent used for applying thethermoplastic resin to the reinforced fibers.
 6. The method of claim 1further comprising the step of backing the forming surface of the toolwith a porous breather lower-density material than the tool between thetool and the exhaust port to reduce the mass of the tool.
 7. The methodof claim 1 wherein the open area of all the passages is approximately1.44% of the total area of the forming surface and the passages aresized to keep resin bleed from the workpiece to a minimum.
 8. A methodfor forming a part on a mold tool, having a rigid forming surfacedefining an overall forming surface area, from a composite material thatemits volatiles during processing, comprising the steps of: laying up avolatile-emitting, fiber-reinforced thermoplastic resin prepreg on arigid forming surface of the mold tool to form a composite workpiece,the tool maintaining dimensional tolerances and having a plurality ofpassages that overlie substantially the entire surface of the workpieceand that extend from the forming surface through the tool, the passagesbeing in fluid contact with an exhaust port and allowing the egress ofvolatiles from the workpiece through the tool when forming the part bycuring the resin, wherein the percentage of open area containing thepassages to the overall forming surface area is approximately 1.44%, thetool also having a backing behind the forming surface comprising aporous breather material that functions to connect the passages in thetool with the exhaust port to reduce the mass of the tool; placing acloth breather material over the tool and the composite workpiece incontact with the passages and placing a vacuum bag around the clothbreather material, wherein the breather material directs the volatilesto the exhaust port; placing a consolidation force on the compositeworkpiece; heating the composite workpiece, while maintaining theconsolidation force, to a temperature at which the composite materialemits volatiles to cure the resin; and removing the emitted volatilesthrough the passages, porous breather, and the exhaust port.